Micro-scale cooling elements of the aforesaid kind are known, and are described e.g. in DE 4 315 580 A1. The micro-scale cooling elements comprise a plurality of individual plies or layers of structured copper films approximately 300 μm thick.
In order to manufacture the micro-scale cooling element, the copper films are layered onto one another in suitable fashion so that the structures that are, for example, etched or stamped into the copper films form a cooling circuit having a micro-scale cooling structure, connecting conduits, and an inflow and outflow.
The copper films are then connected to one another by direct copper bonding, for which purpose oxide layers are created on the surfaces of the copper layers and are then welded to one another.
During operation, a cooling medium such as, for example, deionized water flows through the micro-scale cooling element that has been built up in layers. The use of deionized water as a cooling medium is regarded as advantageous because it exhibits only a small degree of interaction with the material of the micro-scale cooling element.
A problem that is perceived with direct copper bonding is that the connections formed between the oxide layers have little sealing tightness, so that the micro-scale cooling element must have a minimum wall thickness of 400 μm in order to ensure a minimum level of protection from micro-scale cooling element leakage.
Because of the oxide layers and because the structures forming the cooling circuit are for the most part etched into the copper films, the micro-scale cooling element is furthermore susceptible to corrosion.
To avoid this corrosion, it is proposed in the existing art to apply passivating layers, made e.g. of nickel, onto the copper layers. The passivating layer is ablated, however, as a result of the flow of deionized water in the micro-scale cooling element, especially in sharp edge regions, and is carried off by the deionized water. The ions thereby introduced into the deionized water can act as a kind of “breakdown catalyst.” The result is that the corrosion susceptibility of the copper exposed in the ablated regions is in some cases in fact accelerated.
A further problem regarding the use of micro-scale cooling elements made of copper is that copper possesses a coefficient of thermal expansion of approximately 17, whereas a component to be soldered onto the micro-scale cooling element, such as e.g. a high-power diode laser made of gallium arsenide, has a coefficient of thermal expansion of approximately 6.5. Stress and distortion between the micro-scale cooling element and the particular soldered-on component can occur as a result of the differing coefficients of thermal expansion.
To solve this problem, the document DE 195 06 091 A1 proposes to provide ceramic layers between the individual copper layers, and to connect the respective ceramic layers to one another in planar fashion using the direct copper bonding technique. The ceramic layers are provided for that purpose with a copper layer on their external sides, a component to be cooled (in this case a diode laser) being arranged on the topmost layer. To improve thermal conductivity, an insert or a buried layer of a material having extremely high thermal conductivity, for example diamond or T-cBN, is introduced into the ceramic layer below the diode layer arrangement. The use of the ceramic layers is intended to ensure that the substrate exhibits a greatly reduced coefficient of thermal expansion as compared with a substrate or cooling element that is made exclusively or metal and in particular of copper.
As already explained above, both a sealing problem and a corrosion problem affect the arrangement known from the document DE 195 06 091 A1 because of the use of oxide layers. Internal stresses can also arise between the copper and ceramic layers, and can likewise result in leakage.